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To:  All users of the ISAM utility program.

Re:  Recent ISAM changes



     The COBOL utility  ISAM  has  had  several  changes  made  to  it
recently which hopefully will make it's use easier and more efficient.
The changes made are in three general areas:


        1.  New messages from ISAM
        2.  New ISAM error checking
        3.  New switches





1.0  NEW ISAM MESSAGES

     The most apparent change to the user will be several new messages
displayed  by  ISAM at the end of a run.  These messages give the user
information concerning the efficiency of the parameters that the  user
has  given  ISAM.   These messages give detailed information about the
wasted space in both the data and index blocks.  Also,  a  message  is
displayed  for both the data and index blocks informing the user about
the amount of disk space required for each logical block in the file.

     Another apparent change is the new message displayed when a  user
is  doing  an ISAM build with the /B switch.  As seen in the following
example, ISAM now will display a recommended blocking factor  for  the
index and data files.  Please note that this blocking factor is chosen
by ISAM as the most efficient blocking factor as  far  as  disk  space
utilization  is  concerned  and  ISAM makes no attempt to optimize the
amount of core that will be required by  LIBOL  when  processing  this
file.


@ISAM

*=FILE1/B
Mode of input file: S
Mode of data file: S
Maximum record size: 240
Key descriptor: X129.29
Records per input block: 10
Total records per Data block: (Recommended = 53): 25
Empty records per data block: 
Total entries per Index block: (Recommended = 18): 18
Empty entries per index block: 
Percentage of Data file to leave empty: 
Percentage of Index file to leave empty: 
Maximum number of records file can become: 10000
                                                                Page 2


[ISMLOV 2 Levels of index ]
[ISMNDR 500 Data records ]

[ISMWSD Wasted 127. words of 1152. 11.0% wasted space in the Data file.]
[ISMLDE One logical Data block equals 9. physical disk blocks.]
[ISMWSI Wasted 0. words of 128.  in the Index file.]
[ISMLIE One logical Index block equals 1. physical disk block.]
[ISMIBS LIBOL's I/O buffer will require 2816. words (6. Pages) of core.]


*^C


     Another change for TOPS10 users ,  although  not  apparent,  will
make  ISAM  files  much  more efficient.  ISAM has now been changed to
approximate the disk space that will be required  for  the  index  and
data files and asks the monitor for that much in as contiguous a space
as is possible.  This will incur considerably less  overhead  than  if
the  files were fragmented into many smaller chunks all over the disk.
Also, when the user restores his file from backup tapes he should  use
BACKUP  since BACKUP does the same disk space allocation that ISAM now
does.
                                                                Page 3


2.0  NEW ISAM ERROR CHECKING

     In addition the these changes ISAM  now  checks  version  numbers
when  reading  an  indexed  file  (  /P  or  /M ).  If it encounters a
discrepancy, usually indicating file errors caused by  system  crashes
during  file  updating,  it will print an error message indicating the
blocks involved.  These errors are controlled under the /I switch,  so
that  the combination of /P/I (or /M/I ) will give warnings instead of
errors and attempt to continue the packing (or maintenance) operation.

        For the case of a version number error in the index file we have:

@ISAM

*=BADIDX/P

Mode of output file: A
Records per output block: 2

?ISMIVD Index version number discrepancy .
        Reading index file DSK:BADFIL.IDX
        Index level 2. index block 44. entry 2. version number 2. points to
        Index block 10. version number 1. .


        while for a similar error occurring within a data file we get:  


@ISAM

*=BADIDA/P
Mode of output file: A
Records per output block: 2


?ISMDVD Data version number discrepancy .
        Reading data file DSK:BADFIL.IDA from index file DSK:BADFIL.IDX
        Index block 32. entry 3. version number 57. points to
        Data block 121. version number 9. .


        Some of the existing errors will also now give block information:


@ISAM

*=BADREC/P
Mode of output file: A
Records per output block: 2


?ISMKOO keys are out of order
        ABCDLKJXYZ000000122ZIF,.$-+CD
        is after
        ABCDLKJXYZ111111121ZIF,.$-+CD
                                                                Page 4


        Data block 122. version number 58. .






     The terms level, block , entry and version are  as  described  in
section  8.6  in  the COBOL Language Manuals .  Block numbers refer to
128 word disk blocks numbered starting with block 1,  which  has  file
address 0.
                                                                Page 5


3.0  NEW ISAM SWITCHES

3.1  The /Statistics Switch

     A new switch is included to provide the user with statistics when
doing the initial build of a file so that the user may choose the most
efficient blocking factor for the file if ISAM's recommended  blocking
factor   is   unreasonable   because  of  core  usage  or  some  other
consideration.  The user must attempt to balance both core  usage  and
disk  space  in  choosing  a blocking factor.  As an aid in doing this
ISAM will display the total amount of core that LIBOL will require  to
process  the file at the end of it's run.  If this figure is very high
or seems unreasonable then the user should rebuild his file  with  the
/S switch in an effort to choose more appropriate blocking factors for
his file.  The statistics include the blocking factor,  percentage  of
wasted  space in the file if that blocking factor is chosen along with
both the disk space needed and the size of the logical block in  words
of core.  They are given for both the data and index files.

     Following is an example of an ISAM build illustrating the use  of
the /S switch to provide statistics while building.




@ ISAM

@ISAM

*=FILE1/B/S
Mode of input file: S
Mode of data file: S
Maximum record size: 240
Key descriptor: X129.29
Records per input block: 10
Total records per Data block: 
     Records  Disk     Wasted  Core
     /block   Blocks   space   (wds)
        3       1       3.9%    128
        6       2       3.9%    256
        9       3       3.9%    384
        12      4       3.9%    512
        15      5       3.9%    640
        18      6       3.9%    768
        21      7       3.9%    896
        24      8       3.9%    1024
        28      9       0.3%    1152
        31      10      0.7%    1280
        34      11      1.0%    1408
        37      12      1.2%    1536
        40      13      1.4%    1664
        43      14      1.6%    1792
        46      15      1.8%    1920
        49      16      1.9%    2048
        53      17      0.1%    2176
                                                                Page 6


        56      18      0.3%    2304
        59      19      0.5%    2432
        62      20      0.7%    2560
        65      21      0.9%    2688
        68      22      1.0%    2816
        71      23      1.1%    2944
        74      24      1.2%    3072
        78      25      0.1%    3200
        81      26      0.2%    3328
        84      27      0.3%    3456
        87      28      0.5%    3584
        90      29      0.6%    3712
        93      30      0.7%    3840
        96      31      0.8%    3968
        99      32      0.9%    4096
(Recommended = 53): 25
Empty records per data block: 
Total entries per Index block: 
     Records  Disk     Wasted  Core
     /block   Blocks   space   (wds)
        18      1       0.0%    128
(Recommended = 18): 18
Empty entries per index block: 
Percentage of Data file to leave empty: 
Percentage of Index file to leave empty: 
Maximum number of records file can become: 10000

[ISMLOV 2 Levels of index ]
[ISMNDR 500 Data records ]

[ISMWSD Wasted 127. words of 1152. 11.0% wasted space in the Data file.]
[ISMLDE One logical Data block equals 9. physical disk blocks.]
[ISMWSI Wasted 0. words of 128.  in the Index file.]
[ISMLIE One logical Index block equals 1. physical disk block.]
[ISMIBS LIBOL's I/O buffer will require 2816. words (6. Pages) of core.]
                                                                Page 7


3.2  The /Check Switch

     As an aid to checking ISAM files that may be suspect  because  of
system  crashes,  the  /C switch has been added to check the ISAM file
for any normally detectable errors (version,record length, key  order,
duplicate  keys,  etc).  In the manner of /I, /C will issue the errors
as warnings (% instead of ?  ) and will continue the scan until EOF./C
uses  the  same  syntax  and  file  name defaults as /P except that it
produces  no  file  output  and   thus   requires   no   output   file
specifications.

     As an example of /C terminal output we have:



@ISAM

*BADFIL/C

%ISMKOO keys are out of order
        ABCDLKJXYZ000000000000,.$-+CD
        is after
        ABCDLKJXYZ000000006ZIF,.$-+CD
        Data block 7. version number 7. .

%ISMIVD Index version number discrepancy .
        Reading index file DSK:BADFIL.IDX
        Index level 2. index block 44. entry 2. version number 2. points to
        Index block 10. version number 1. .

%ISMRTS record too short to contain key field
        Data block 89. version number 25. .

%ISMDPK two keys with equal value = ABCDLKJXYZ000000088ZIF,.$-+CD
        Data block 89. version number 25. .

%ISMIVD Index version number discrepancy .
        Reading index file DSK:BADFIL.IDX
        Index level 2. index block 67. entry 3. version number 13. points to
        Index block 32. version number 219. .

%ISMDVD Data version number discrepancy .
        Reading data file DSK:BADFIL.IDA from index file DSK:BADFIL.IDX
        Index block 32. entry 3. version number 57. points to
        Data block 121. version number 9. .

%ISMKOO keys are out of order
        ABCDLKJXYZ000000122ZIF,.$-+CD
        is after
        ABCDLKJXYZ111111121ZIF,.$-+CD
        Data block 122. version number 58. .

*^C
                                                                Page 8


3.3  The /Rename Switch

     The current ISAM facility within LIBOL obtains the name  of  data
file portion of the ISAM file (usually an .IDA file ) from the name as
it is saved in the statistics portion of the index file ( usually .IDX
file  ).   For this reason the normal monitor RENAME commands will not
correctly rename the ISAM file pair.  To make the RENAME possible, the
/R  switch  has  been added to ISAM.  It uses standard ISAM syntax and
defaults , as in:

        new-index-file,new-data-file=old-file/R


     As an example we have:

@DIRECTORY (OF FILES) *.I*

   CBL20:<HAM-TST>
 BADFIL.IDA.1
   .IDX.1

 Total of 2 files
@ISAM

*GOODFIL=BADFIL/R

*^C
@DIRECTORY (OF FILES) *.I*

   CBL20:<HAM-TST>
 GOODFI.IDA.1
   .IDX.1

 Total of 2 files

@;NOTE THAT ISAM TRUNCATES NAMES TO SIX CHARACTERS!!!

@ISAM

*BADFIL.IX1,BADFIL.IX2=GOODFI/R

*^C
@DIRECTORY (OF FILES) *.I*

   CBL20:<HAM-TST>
 BADFIL.IX1.1
   .IX2.1

 Total of 2 files
@ISAM

*BADFIL=BADFIL.IX1/R

*^C
@DIRECTORY (OF FILES) *.I*
                                                                Page 9



   CBL20:<HAM-TST>
 BADFIL.IDA.1
   .IDX.1

 Total of 2 files
                                                               Page 10


3.4  The /ADVancing Switch

     The /ADV switch may be used to specify the  advancing  mode  when
outputing   ASCII   sequential   files.   The  switch,  which  may  be
abbreviated /A or /AD, has the following syntax:



        sequential-file/ADV:mode = index-file/P


        where mode =    6[8]            - use COBOL-68 default  (BEFORE)
                        7[4]            - use COBOL-74 default  (AFTER)
                        A[FTER]         - AFTER ADVANCING 1 LINE
                        B[EFORE]        - BEFORE ADVANCING 1 LINE




     When not specified with the /ADV switch, ADVANCING will default